The world depends on hydrocarbons to solve many of its energy needs. Consequently, oil field operators strive to produce and sell hydrocarbons as efficiently as possible. Much of the easily obtainable oil has already been produced, so new techniques are being developed to extract less accessible hydrocarbons. These techniques often involve drilling a borehole in close proximity to one or more existing wells. One such technique is steam-assisted gravity drainage (“SAGD”) as described in U.S. Pat. No. 6,257,334, “Steam-Assisted Gravity Drainage Heavy Oil Recovery Process”. SAGD uses pairs of horizontal wells spaced less than about 10 meters apart, and careful control of the spacing is important to the technique's effectiveness. Other examples of directed drilling near an existing well include intersection for blowout control, multiple wells drilled from an offshore platform, and closely spaced wells for geothermal energy recovery.
The detection of a nearby well is usually insufficient. Drillers often require an accurate determination of distance and direction to the nearby well, which determinations typically require the use of one or more azimuthally-sensitive tools. Such tools in turn require an orientation-monitoring mechanism, which usually includes one or more magnetometers whose measurements may be subject to interference from stray magnetic fields from, e.g., magnetization of the well casing or the presence of ferromagnetic minerals in the formation. Such interference has hereto been regarded as negligible and generally ignored, but the present inventors have determined that ranging direction and distance determinations can be substantially affected by such interference to the driller's detriment.